[css-typed-om] update CSSPerspectiveValue to accept perspective(none)

[dbaron]

In w3c/csswg-drafts#6488 the CSS Working Group resolved to add perspective(none) and it was added in w3c/csswg-drafts#6691 .

This requires updates to CSSPerspective.

It's not clear to me if the best way to make these updates is by changing CSSNumericValue to CSSStyleValue or changing it to (CSSNumericValue or CSSKeywordValue), or perhaps (although probably not) to CSSNumericValue?.

Maybe @tabatkins or others have opinions on this?

[tabatkins]

It should be (CSSNumericValue or CSSKeywordValue); that's how it'll be reflected from syntax into Typed OM.